The art of Process Control pt 1- Plates
The printing industry is technically a section of the manufacturing industry. Instead of manufacturing tires or pipes or cars, we manufacture ink on paper. In manufacturing, you need process controls to insure that every step of the process is controlled, minimizes waste, maximizes profit and delivers the best quality product to the customer.
Process control is an integral part of printing as well, and it is more of an art form than an exact science. I am going to look at what is involved in process controls on the press side of printing- from plates to paper. Some of these subjects get pretty complex, so I may just gloss over certain things, or add links to an outside reference if you are interested.
The first step is to check your plates. In offset lithography, the plate is one of the most important parts of the process. When the ink touches the plate, it sticks to the image area and is then transfered to the blanket and then the paper, so if you have bad dots on your plate then everything else will be suffer.
A special measurement device is needed to measure a plate. This device measures the screen that is imaged onto the plate. So if we send a 20% screen to the platemaker, the plate should have a 20% screen on it. Since plates are imaged either with a UV lamp or with a type of laser, then processed chemically like a photograph, typically a curve must be applied to the data to make a plate linear. A linearized plate means that when we send a 10-20-30% screens to the platemaker, the plates have the same screens on output.
Once plates are linearized we need to be able to easily monitor them. Processing chemistry can change, age of bulbs, dust, and laser calibration can all affect the image on a plate. Most modern CTP systems can automatically put an exposure target in a non-image area of the plate. This target is a quick visual indicator for how the plate was processed. So as long as the exposure target, which imaged and processed at the same time the rest of the plate is, looks the way it should, then the plate should be fine.
Once you know that your plates are outputting correctly, and to a known and controlled standard, it is time to move on to the press.